1. Field of the Invention
The present invention relates to a process for preparing C.sub.4 -C.sub.6 -alkenes having an internal double bond by hydroisomerization of C.sub.4 -C.sub.6 -alkenes having a terminal double bond, in which the product stream exiting from the reactor is divided into a work-up stream and a recycle stream, and the recycle stream is recycled to the reactor inlet, where, together with the C.sub.4 -C.sub.6 -alkenes having a terminal double bond and the H.sub.2, it forms the feed for the reactor. The process serves especially for converting 1-butene into 2-butene.
2. Description of the Related Art
Alkenes having an internal double bond, for example 2-butene, are sought-after alkylating agents for alkylating n-alkanes and/or i-alkanes, valuable motor fuels ("alkylate gasoline") being produced. Such alkylates of olefins having an internal double bond produce, in the motor fuel sector, better properties than alkylates of olefins having a terminal double bond. Thus, for example, n-1-butene with n-butane/i-butane produces an alkylate gasoline having a research octane rating ROR of 92.5, whereas the corresponding alkylate of 2-butene has an ROR of 98.5 (Oil and Gas Journal 8 (1971), 60). The shift of terminal double bonds to an internal position of the olefin therefore represents an important step in the production of high-octane gasoline.
The conversion, for example, of 1-butene to 2-butene has long been carried out as a so-called hydroisomerization in the presence of hydrogen on a catalyst suitable for the hydrogenation. Thus U.S. Pat. No. 3,531,545 discloses using for this purpose a catalyst having a content of Pd or Pt on SiO.sub.2 or Al.sub.2 O.sub.3 as support, which, however, to avoid the undesirable overhydrogenation, is damped in its activity using sulfur compounds. However, as a consequence of this, the hydroisomerization must be carried out at a temperature of up to 155.degree. C. However, the thermodynamic equilibrium is more on the side of 1-butene at higher temperatures, so that lower hydroisomerization yields can be achieved at higher temperatures. According to FR 2,438,084, an increase in selectivity of a Pd/Al.sub.2 O.sub.3 catalyst can also be induced, apart from a treatment with sulfur compounds, by a treatment with ammonia or carbon monoxide. A further variant of the increase in selectivity is described in U.S. Pat. No. 4,132,745 in which a Pd/Al.sub.2 O.sub.3 catalyst is treated with hydrogen sulfide and then with hydrogen; at 80 to 100.degree. C., using such a catalyst, a hydroisomerization is carried out in which the accompanying butadiene is substantially eliminated by hydrogenation, but a considerable portion of butene is also undesirably hydrogenated to give the saturated butane. DE-A 31 40 573 describes a hydroisomerization of 1-butene to 2-butene on a catalyst containing 0.3% Pd on high-purity Al.sub.2 O.sub.3, which is performed subsequently to the oligomerization of i-butene and in the presence of the oligomer. The isomerization is carried out at 120.degree. C. and at a molar ratio of hydrogen to hydrocarbons of 0.5. EP-A 338 309 discloses that alkenes having a terminal double bond can be isomerized to form alkenes having an internal double bond if the catalyst used is a macroporous or gel-form cation exchanger in the H+ form, which contains 0.001 to 10 g of a metal of subgroup VIII of the Periodic Table of the Elements (Mendeleev) per liter of cation exchanger.